1. Field of the Invention
This invention relates to a finder for single-lens reflex type digital cameras which uses an electronic image sensor such as a CCD.
2. Description of Related Art
In recent years, special attention has been devoted to a digital camera (electronic camera) as an alternative, used in the next generation, to a 35 mm silver halide film (usually called a Leica size) camera. The size of a CCD used as an image sensor in this digital camera varies in diagonal length from a fraction to the order of tenth of a 35 mm silver halide film. Thus, where a single-lens reflex system is applied to the camera of this type, it is necessary to increase the magnification of an ocular optical system used in a finder system. In doing so, the focal length of the finder system must be reduced as a matter of course. Furthermore, it is also necessary to interpose an image erecting optical system between the first imaging plane and the ocular optical system of the finder system.
However, the pupil diameter and the eyepoint position of the finder system must be constant, irrespective of the focal length of the ocular optical system of the finder system, and a sufficiently wide field angle must also be secured for a single-lens reflex camera. As such, an effective sectional area of the image erecting optical system tends to increase. In order to construct the image erecting optical system with a prism under such circumstances, its optical path length must be increased. This makes the power distribution of the ocular optical system very difficult and develops a tendency that aberrations affecting the view of the finder are considerably deteriorated.
Consequently, in order to solve the above problems, it is necessary to take account of a prism design prepared to have the shortest possible optical path length and the construction of an ocular optical system with a power distribution and a lens configuration which are advantageous for correction for aberrations.